C. Davison — Vorticose Earthquake Shocks. 257 



No evidence of any series of changing conditions such as a succes- 

 sion of cold periods, or an age of depression, has been discovered, but 

 all the glacial phenomena of the district must be referred to one and 

 the same glacial period when the country was covered with snow 

 and local glaciers. The sands and gravels are either distinctly of 

 fresh-water origin (as the Rheidol river gravels), or, as in the case 

 of the Esker Eidge of Llanrhysted Road, of doubtful origin ; and this 

 latter I refer to the combined action of water and ice, the accumu- 

 lation of ice causing special drainage conditions determining the 

 position and influencing the construction of the deposit. 



III. — On the Theory of Vorticose Earthquake-Shocks. 

 By Charles Davison, B.A., Late Scholar of Emmanuel College, Cambridge. 



EARTHQUAKE-SHOCKS have generally been divided into 

 three classes : 

 a. Undulatory shocks, by far the most commonly felt, consisting 

 of one or several waves, tlie movement at any place being to and 

 fro along a line inclined to the horizon, but in general nearly 

 horizontal. 



&. " Sussultatore " shocks, consisting of one or more up-and-down 

 movements, in a nearly vertical direction. 



c. Vorticose shocks, consisting sometimes of a true rotatory motion, 

 but generally, it would seem, of undulatory or " sussultatore " move- 

 ments crossing one another in different directions, so that, it has 

 been said, the surface of the ground appears like the irregular surges 

 of a troubled sea. 



It is with the last of these, the vorticose shocks, that we intend 

 to deal in this paper ; we shall also, however, point out one or two 

 conclusions which may be drawn from their occurrence, and briefly 

 state the connexion that subsists between the three kinds. 



Nature of Vorticose Shocks. — The following descriptions of some 



vorticose shocks may be given in illustration of the above definition : 



1845, Aug. 16, at Ragusa (Dalmatia), an earthquake, preceded and 



followed by loud subterranean noises ; it was at first undulatory, 



afterwards vertical ; it lasted 8 seconds.^ 



1858, Aug. 6, at Oneglia, an earthquake, which was at first vertical, 



afterwards undulatory from N. to S., and of 8 seconds duration.^ 

 1812, March 26, at Caraccas. The first shock of this disastrous 

 earthquake lasted 5 or 6 seconds. Immediately after this there 

 was a second shock of from 10 to 12 seconds duration, during 

 which the ground seemed to boil. Afterwards a subterranean 

 noise louder than thunder preceded, by about 3 or 4 seconds, a 

 vertical movement, followed by an undulatory movement, in which 

 the shocks were from N. to S. and from E. to W.^ 



^ M. Perrey. " Liste les tremblements de terre ressentis pendant les annees 1845 

 et 1848 " : Mem. de I'Acad. de Dijon, 1845-46. 



2 M. Perrey. "Note sur les tremblements de terre en 1858, etc." : Mem. Coux. 

 de I'Acad. Eoy. de Belgique (1861), Collection in 8vo. tome xii. 



^ M. Perrey. "Note historique sur les tremblements de terre des Antilles": 

 Comptes Eendus (June 12, 1843), vol. xvi. pp. 1283-1303. 



DECADE II. — TOL. IX. NO. VI. 17 



